[PATCH RFC v3 10/43] KVM: guest_memfd: Handle lru_add fbatch refcounts during conversion safety check

[Ackerley Tng <ackerleytng@google.com>]

When checking if a guest_memfd folio is safe for conversion, its refcount
is examined. A folio may be present in a per-CPU lru_add fbatch, which
temporarily increases its refcount. This can lead to a false positive,
incorrectly indicating that the folio is in use and preventing the
conversion, even if it is otherwise safe. The conversion process might not
be on the same CPU that holds the folio in its fbatch, making a simple
per-CPU check insufficient.

To address this, drain all CPUs' lru_add fbatches if an unexpectedly high
refcount is encountered during the safety check. This is performed at most
once per conversion request.

guest_memfd folios are unevictable, so they can only reside in the lru_add
fbatch. If the folio's refcount is still unsafe after draining, then the
conversion is truly deemed unsafe.

Signed-off-by: Ackerley Tng <ackerleytng@google.com>
---
 virt/kvm/guest_memfd.c | 22 ++++++++++++++++------
 1 file changed, 16 insertions(+), 6 deletions(-)

diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c
index 8e4866bb8145d..c4f6bdad6289e 100644
--- a/virt/kvm/guest_memfd.c
+++ b/virt/kvm/guest_memfd.c
@@ -8,6 +8,7 @@
 #include <linux/mempolicy.h>
 #include <linux/pseudo_fs.h>
 #include <linux/pagemap.h>
+#include <linux/swap.h>
 
 #include "kvm_mm.h"
 
@@ -566,25 +567,34 @@ static bool kvm_gmem_range_has_attributes(struct maple_tree *mt,
 	return true;
 }
 
-static bool kvm_gmem_is_safe_for_conversion(struct inode *inode, pgoff_t start,
-					    size_t nr_pages, pgoff_t *err_index)
+static bool kvm_gmem_is_safe_for_conversion(struct inode *inode,
+					    pgoff_t start, size_t nr_pages,
+					    pgoff_t *err_index)
 {
 	struct address_space *mapping = inode->i_mapping;
 	const int filemap_get_folios_refcount = 1;
 	pgoff_t last = start + nr_pages - 1;
 	struct folio_batch fbatch;
+	bool lru_drained = false;
 	bool safe = true;
 	int i;
 
 	folio_batch_init(&fbatch);
 	while (safe && filemap_get_folios(mapping, &start, last, &fbatch)) {
 
-		for (i = 0; i < folio_batch_count(&fbatch); ++i) {
+		for (i = 0; i < folio_batch_count(&fbatch);) {
 			struct folio *folio = fbatch.folios[i];
 
-			if (folio_ref_count(folio) !=
-			    folio_nr_pages(folio) + filemap_get_folios_refcount) {
-				safe = false;
+			safe = (folio_ref_count(folio) ==
+				folio_nr_pages(folio) +
+				filemap_get_folios_refcount);
+
+			if (safe) {
+				++i;
+			} else if (!lru_drained) {
+				lru_add_drain_all();
+				lru_drained = true;
+			} else {
 				*err_index = folio->index;
 				break;
 			}

-- 
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